In-situ leaching, also called in-situ recovery or solution mining, is traditionally a process of recovering minerals such as copper and uranium through boreholes drilled into the deposit. The process initially involves drilling of holes into the ore deposit, while explosive or hydraulic fracturing may be used to create open pathways in the deposit for solution to penetrate. Leaching solution is pumped into the deposit where it makes contact with the ore. The solution bearing the dissolved ore content is then pumped to the surface and processed. This process allows the extraction of metals and salts from an ore body without the need for conventional mining involving drill-and-blast, open-cut or underground mining.
Conventional solution mines create individual caverns, usually by dissolving salt from beneath the ore body, then rubblizing the ore into the cavern and dissolving the ore in fresh water or dilute brines to form near saturated solutions at temperatures equal to the ore temperature (or slightly higher). Caverns tend to develop vertically and, in some cases, consideration has been given to connecting caverns. In order to collect the ore from the solution, crystallization systems are necessary at the surface.
Conventional solution mining systems have difficulty raising the mine temperature above the formation temperature, as well as obtaining fully saturated brines. Thus at the surface they raise the potash concentration and temperature in evaporators. This is the most expensive part of the plant using large amounts of expensive and exotic metals. Large amounts of steam are also required in this process. The hot concentrated brines are then crystallized in evaporative crystallizers; these are limited to cooling to about 25 degrees C. One mine uses a system which takes the cooled brine to ponds, and makes use of natural cold crystallization in open ponds to add to plant recovery. This is still an expensive process requiring careful management and expensive dredging equipment. It also is seasonal with no potential to recover any heat.
Potash has been mined by solution mining techniques developed in the 1960s, as demonstrated at the Mosaic Belle Plaine Mine in Saskatchewan, Canada. The established method uses well pairs, from 50 to 80 meters apart.
In the predevelopment stage, water is pumped into the individual wells. Each well is equipped with a double casing. Water is pumped down the centre string, with brine returning up the annulus. When the caverns from each well connect due to dissolution of intervening materials, the water will then be pumped down one well and produced to surface through the second well to continue to wash out a salt cavern. This predevelopment cavern is commonly referred to as a sump and is located under the lowest potash bed. The water/brine is overlaid by oil or diesel fuel to prevent dissolving the overlying potash layer. The salt brine from this predevelopment stage is conventionally pumped to a deep well for disposal.
Primary mining commences after the sump is developed. Layers of the ore are broken into the sump (rubblized). In primary mining, preheated water is slowly pumped into the cavern to dissolve the potash and salt in the ore. When the brine comes to a desirable potash concentration (typically somewhere above 10% KCl, and about 18% NaCl), water flow to the cavern is set (commonly at around 50 cubic meters per hour) to maintain this discharge concentration throughout the primary mining stage, until the whole ore layer is removed. The brine temperature coming from the well is close to the ore temperature since the low flow rate limits the amount of heat that can be added even if the feed water is very hot. The slow dissolution rate limits flow to and from the cavern. A large scale mine will require as many as 40 well pairs, 2 per cavern (80 wells), at a given time.
While the brine is technically close to saturation, the potash level is lower than equilibrium, while the salt concentration is typically higher than at equilibrium conditions. In fact, the ratio of KCl to NaCl in the brine must be in the ratio of the KCl to NaCl ratio in the ore body since primary mining is generally defined as full dissolution of the ore. The unfavorable brine concentration requires that brine from the wells must be fed to expensive evaporators, then to crystallizers to recover the potash. Over 1 tonne of salt is produced in evaporation for each tonne of potash, and this salt is produced and stored on surface in large salt piles, with no end use.
About 30% of the production from a conventional solution mining operation is from secondary mining. A hot, NaCl saturated brine replaces the water feed used in primary mining. When the brine comes to near saturation, the new mixed KCl/NaCl brine from the mine is cooled in contact crystallizers or cooling ponds to produce the previously dissolved potash. This is an even slower process than with primary mining, and is seasonal when cooling ponds are used (only in cold winter weather).
It has long been suspected that if brine could be produced in the mine close to an equilibrated concentration, at elevated temperature (above 50 degrees C. but preferably closer to 80 degrees C.), this brine could simply be cooled in a crystallizer to produce a crystal potash product. This would eliminate, or at least minimize, the need for an expensive evaporation stage. It was believed that if the cold brine from the crystallizer is reheated and used for mine feed, only the potash will dissolve, while all the salt will be retained in the cavern and there will be no salt waste stored on surface.
Though this idea was well established, every attempt to implement it resulted in very limited production lasting only days. The liberated salt quickly filled the lower portion of the cavern, and dissolution rates continued to decline. Most tests were run using saturated NaCl brine, since crystallizers were not included in the test facilities, and the NaCl saturated brines blinded off the deposit. Tests were also done on single wells with the feed brine going down the centre string, and potential production brine rising in the annulus. This limited circulation to the deposit, however, and made heating of the deposit very difficult since the hot feed was cooled by the returning production brine by heat exchange across the tubing wall.
There have been limited attempts to overcome the problems with the conventional solution mining method. One example is Canadian Patent No. 2,725,013, owned by the present assignee, which provides a novel solution mining method that has advantages over the conventional technique. It teaches a method for the use of curved flow patterns to allow extraction of the potash values while leaving the undesirable salt in the cavern. This, then, allows polythermic mining of potash using a simple plant with cooling crystallizers, centrifuge and dryer as primary equipment. Alternately, a cooling pond could be used alone or in combination with conventional crystallization equipment or wiped surface crystallizers.
What is needed is a simplified well layout that has advantages for large scale production, without evaporation equipment or salt tailings stockpiles at surface. An improved solution mining method is presented in the following.